Make test workflow a cron job

Author: honno

.github/workflows/test.yml

@@ -1,5 +1,8 @@
 name: Run tests
-on: [push]
+on:
+  push: null
+  schedule:
+    - cron: '0 0 * * 0'  # i.e. every week
 jobs:
   test:
     runs-on: ubuntu-latest